[various] Fixes unawaited_futures violations (#4067)
This option had been disabled to match flutter/flutter, but the reason it was disabled there was "too many false positives", mostly around animation. That doesn't apply to most packages here, and we've had a number of production bugsâespecially in plugins, that use async heavily in ways that are intended to be client-awaitableâthat this would have caught.
This PR:
- Enables the option at the repo level.
- Permanently (unless the owners decide to change it) opts out animations and go_router, both of which looked like mostly or entirely false positives.
- Temporarily opted out a few plugins that have a lot of violations that should be handled in their own PRs later (camera_android_camerax, most of webview_flutter).
- Fixes all remaining violations.
In many cases this PR is behavior-changing, replacing implicitly unawaited futures that did not seem obviously intentional with awaited futures, so non-test code in particular should be reviewed carefully to make sure the changes are correct. All of the changes are manual, not fix-generated.
Part of https://github.com/flutter/flutter/issues/127323